<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <script>
    let str= 'abcacd'
 
    const fn= (str)=>{
      console.log(str.split('').filter((item,index,arr)=>{
        return str.indexOf(item)===str.lastIndexOf(item)
      })[0]);
    }
 fn(str)

  </script>
</body>
</html>